This invention pertains to a composite extruded structure comprising a continuous resin sheath and an inner core comprising a substance whose melting point or decomposition temperature is lower than the melting termperature of the thermoplastic material of the sheath and to the process and apparatus for making such structure.
Composite structures consisting of a core sheathed with thermoplastic material are known; these generally involve materials having metal cores, for example, as in the case of electric cables or wires. Techniques adapted for such types of manufacture, such as that described in U.S. Pat. No. 3,764,642, make it possible to manufacture only composite materials wherein the core is insensitive not only to the melting temperature of the material constituting the sheath, but also to that of the extrusion die. This is also the case for French Patent No. 2,021,601, which describes a die making it possible to sheath a very long thermoplastic material which has a melting point higher than that of the sheath (page 8, lines 28-30) and is thus not fragile at the extrusion temperature of said sheath. These composites are usually manufactured by extrusion by having the wire, which in most cases is metal, pass through the sheathing die. With such a method, it is thus entirely impossible to manufacture a composite whose core cannot tolerate the extrusion temperature of the sheath.
According to European Patent No. 0133825, it is only possible to obtain a composite structure wherein the melting temperature of the core is at best equal to that of the sheath; i.e., a material having a core which is not sensitive to the sheathing temperature under known operating conditions.
U.S. Pat. No. 3,239,884 describes a die tool which makes it possible to sheath a cable which is sensitive to thermal effects. The apparatus making it possible to sheath a cable consists of a die tool represented by a tubular die and a punch and a tubular cable guide. This guide fitted with an internal cooling device is placed in axial alignment with the outlet land of the die tool which forms the internal wall of the sheath, while keeping open a channel which allows the circulation of compressed air or allows the creation of a vacuum between the cable, the guide, and sheath, with the interface between the cable and the sheath ensuring sealing and continuity between the guide conduit and the channel.
According to this apparatus, if a stream of air is used as the cooling means, it comes directly into contact with the interface between the cable and sheath. Direct impingement of the cooling stream is not a problem when only a single cable is being sheathed: all that is necessary is to control it properly so as not to disturb the sheathing process.
The situation is entirely different when a core which is particularly fragile, either thermally or mechanically, is being sheathed, as, for example, in the case of sheathing extremely fine-fiber rovings and/or powders. In the case of the apparatus according to said U.S. Pat. No. 3,239,884, it is not possible to ensure control of the airflow in cases where fiber rovings are being sheathed, which leads to irregularities in the formation of the sheath. The same applies in the case where powders are being sheathed, since they tend to back up either in the guide or in the air-outlet channel depending on the direction of the airflow, inevitably leading to blockage at some point in the apparatus, which at best produces breakage of the extruded material and at worst leads to decomposition of the materials due to blockage of the die.